The field of fibre-optic transmission and reception is on the verge of massive penetration of the electrical communications market. Due to the low attenuation characteristics of optical fibres over the relatively wide frequency bands of transmission, relatively large repeater spacings and hence substantially reduced costs are possible. A decrease in attenuation of fibres by a few decibels per kilometer means a corresponding increase in distance between repeaters. Of course, there exists a theoretical limit to the reduction in the attenuation, and, surprisingly, some of the latest fibres are quite close to this theoretical limit. Any further improvements in repeater spacing will have to, sooner or later, be sought elsewhere.
As is well understood in the art of electrical communications, an improvement in signal-to-noise ratio (SNR) at the receiving end of a transmission system is equivalent to, and as desirable as, a reduction in the attenuation of that transmission path. An increase in SNR of 3 decibels in a system having a transmission path exhibiting, say, 7 decibels per kilometer would, theoretically, permit an increase in repeater spacing of up to 0.2 kilometer. This, of course, provided that the receiving amplifier directly connected to the photoelectric converter (often a PIN diode) is the limiting factor in SNR determination. While this is the case with PIN photodiodes, it is not the case with avalanche photodiodes, which are limited by internal shot noise. Nevertheless, an advantage still accrues with avalanche photodiodes due to lower avalanche gain requirements due to the effective decrease in amplifier noise.
The conventional way of connecting a photodetector to its associated receiver-amplifier is by tapping the junction between the photodetector and its load resistor. Photons impinging on the reverse-biased detector generate a photocurrent proportional to their intensity, which current flows through the single load resistor and develops a proportional photovoltage thereacross.